Overview
Xubuntu is a Canonical Ltd.–recognized, community-maintained derivative of the Ubuntu operating system. The name Xubuntu is a portmanteau of Xfce and Ubuntu, as it uses the Xfce desktop environment, instead of Ubuntu's customized GNOME desktop.
Xubuntu seeks to provide "a light, stable and configurable desktop environment with conservative workflows" using Xfce components. Xubuntu is intended for both new and experienced Linux users. Rather than explicitly targeting low-powered machines, it attempts to provide "extra responsiveness and speed" on existing hardware.
History
Xubuntu was originally intended to be released at the same time as Ubuntu 5.10 Breezy Badger, 13 October 2005, but the work was not complete by that date. Instead the Xubuntu name was used for the xubuntu-desktop metapackage available through the Synaptic Package Manager which installed the Xfce desktop.
From Wikipedia (CC BY-SA 4.0).